1) Field of the Invention
The invention relates to a method for the commutation of a brushless direct current motor according to the preamble of the independent patent claims.
2) Description of the Related Art
In a brushless DC motor, the motor current is commutated in specific positions of the rotor by electronic switching elements such as transistors. In a single-strand motor, this occurs two times per electric rotation, the voltage applied to the single winding changing poles alternately. In a three-strand motor, commutation occurs every 120° (electric). To ensure that this commutation process takes place at the correct point in time, the momentary position of the motor has to be known.
The usual method of registering the position of the motor is based on the use of Hall sensors. These components are generally arranged in the neutral zone(s) of the armature circumference and measure the magnetic field generated by the permanent magnet rotor. Neutral zones are any points on the armature circumference in which the normal components of induction are zero, in other words, the middle of the stator slot. When the rotor reaches a neutral zone, which also corresponds to the mechanical commutation position, the Hall sensor generates a commutation signal.
Since the motor winding has relatively high inductance, the motor current does not reach its maximum immediately after being switched on but rather lags behind the applied motor voltage, that is, it is delayed by a certain period of time. In order to compensate for this delay, pre-commutation is carried out. This means that the commutation process is started before the rotor has reached the actual commutation position.
There are essentially two ways in which pre-commutation can be realized. One way is for the Hall sensor not to be placed in the neutral zone but rather in a position slightly before the neutral zone in the direction of rotation of the rotor. This results in the Hall sensor generating a commutation signal before the rotor reaches its actual commutation position. This solution is suitable for motors which only operate in one rotational direction, such as motors to drive fans. The disadvantage of this method is that here the magnetic field generated by the stator current can influence the measurement signal of the Hall sensor which can lead to start-up difficulties for the motor, among other problems.
Another method of pre-commutation makes use of the fact that motors operating under a load, particularly in fan applications, often have a high moment of inertia. Because of this, the operating speed changes relatively slowly when the operating conditions change. This inertia is exploited during commutation in that the time between two changes in polarity in the Hall signal, that is the time between two successive commutation signals, is measured. If we assume that the time measured between the next two changes in polarity remains constant, the time of pre-commutation can be calculated from the measured time and measured by means of a timer. This timer can either take the form of a hardware component, e.g. the counter/timer unit of a microcontroller, or be realized in software form. The advantage of this method is that the Hall sensor can be placed in the neutral zone where, due to reasons of symmetry, the influence of the winding current on the sensor is minimal. The disadvantage is the slight increase in requirements placed on the control electronics.
There are also sensor-less motors which do not have any separate position sensors, that is, the position of the rotor is determined without the use of Hall sensors or generally without using any special position sensors. In such sensorless motors, the motor windings are used as sensors. Here, the position of the rotor is derived from electrically measurable parameters (motor voltage and/or motor current). This method is particularly advantageous for motors with external commutation electronics since here not only can the sensors and their supports in the motor, if any, be omitted, but also the sensor leads to the external electronics and the related plugs.
Irrespective of how the rotor position is measured and how pre-commutation is realized, the question remains as to how large the angle of pre-commutation need be. Changes in this angle can influence not only the torque-speed characteristics of the motor but also its efficiency. If the commutation angle is realized by means of displacing the Hall sensors, the pre-commutation angle can only be optimized for a specific operating point, since the angle itself is established by the mechanical angular position of the Hall sensors. If, however, pre-commutation is realized in the form of software, the commutation angle can be adjusted to the required operating point.